This disclosure relates in general to electrostatography and, more specifically, to an electrostatographic imaging member having a charge transport layer comprising a stereo-hindered charge transport material containing bulky organic groups, and a method for protecting the nitrogen atoms of charge transport materials and, thus, the charge transport layer from chemical damage.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional protective or overcoating layer(s). The imaging members can take several forms, including flexible belts, rigid drums, etc. For many multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the backside of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having a separate charge generating (photogenerating) layer (CGL) and charge transport layer (CTL). The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors includes, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive materials may be formed as a continuous, homogeneous photogenerating layer. The disclosure of U.S. Pat. No. 4,265,990 is incorporated herein by reference.
Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384, 4,306,008, 4,299,897 and 4,439,507. The disclosures of these patents are incorporated herein in their entirety. Charge transport layers are known to be comprised of any of several different types of charge transport material dispersed in a polymer binder.
U.S. Pat. No. 4,806,443 describes a charge transport layer including a polyether carbonate (PEC) obtained from the condensation of N,N′-diphenyl N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine and diethylene glycol bischloroformate. U.S. Pat. No. 4,025,341 similarly describes that a photoreceptor includes a charge transport layer including any suitable hole transporting material such as poly(oxycarbonyloxy-2-methyl-1,4-phenylenecyclohexylidene-3-methyl-1,4-phenylene.
In multilayer photoreceptor devices, one property, for example, is the charge carrier mobility in the transport layer. Charge carrier mobility determines the velocities at which the photo-injected carriers transit the transport layer. For greater charge carrier mobility capabilities, for example, it may be necessary to increase the concentration of the active molecule transport compounds dissolved or molecularly dispersed in the binder. Phase separation or crystallization sets an upper limit to the concentration of the transport molecules that can be dispersed in a binder.
In conventional layered organic photoreceptor devices, the charge transport layer (CTL) generally includes an activating compound dispersed in electrically inactive polymeric materials, thereby making the polymeric materials electrically active. Generally, the charge transport layer includes charge transport small molecules (CTM) and polymer binders. Typical inactive resin binders include polycarbonate resin, polyether carbonate, polyester, polyarylate, polyether, polysulphone, and the like. The charge transport molecules typically include aromatic amine compounds such as N N′, diphenyl-N,N′-di(m-tolyl)-p-benzidine [TPD].
The charge transport layer, however, is a thin coating layer of usually less than 40 microns. Because the charge transport layer is thin and there are no special chemical interactions between the charge transport molecules and the polymeric binders, the charge transport layer generally exists in a meta-stable state. Consequently, these kinds of charge transport molecules are susceptible to crystallization, which causes physical damage to the charge transport layer such as phase deformation, and low wear resistance.
Further, the small charge transport molecules, and particularly the nitrogen atoms of the charge transport molecules, may be easily oxidized by ozone, nitric oxide and other oxidative materials generated under normal operational conditions. In this regard, the nitrogen atom, connected with conjugated molecular structures like double bonds and/or benzene rings, is the core for charge transport materials. Consequently, the protection of nitrogen from chemical change is very important for organic photoreceptor devices.
These problems may be overcome in some degree by the use of a second transport layer having a higher concentration of donor or acceptor molecules, the use of a single layer with a concentration gradient, or the incorporation of antioxidant materials into the transport layer. The electrical properties of devices employing such methods, however, are often significantly compromised.
What is still desired is an improved material for a charge transport layer of an imaging member that exhibits excellent performance properties and has the further advantages of not being susceptible to crystallization, oxidation or chemical attack/damage when present in the charge transport layer.
These and other non-limiting aspects and/or objects of the development are more particularly disclosed below.